As we have recently learned, Advanced Busoshoku Haki can be emitted a short distance away from the user’s body without a medium. This is first seen when Sentomaru deflected Luffy’s attack without making direct contact.
Advanced Busoshoku involves channeling excess Haki from around their body through their fists. This allows the user to repel an enemy without making direct contact or using brute force. So far Kuzan, Sakazuki, Borsalino, Sentomaru, Rayleigh, and Hyogoro have been seen using it.
Luffy believes that Advanced Busoshoku might be powerful enough to injure the extremely durable Kaido, who was able to easily shrug off regular Busoshoku when Luffy attacked him with Gear third and even Gear fourth.
In chapter 947 the concept of Advanced Busoshoku Haki is further explained.
An even higher grade of this technique causes the user’s Haki to reach inside the target’s body and destroy them from within.
This level has only been utilized by Rayleigh and Luffy, which the latter did unconsciously. Both used it to destroy collars that were designed to kill a person under certain circumstances.
Luffy recognizes that what he did to remove his and Hyogoro’s death collars was the same technique that he saw Silvers Rayleigh use to remove Camie’s exploding collar on Sabaody two years ago. However, since he used it in the heat of the moment, he is not sure how to activate it at will.
Hyogoro notes that what Luffy just did was superior to what he had been trying to teach him, and he tells Luffy that there is another step in concentrating and projecting Haki, which is the ability to destroy nearby objects from the inside. This is what Luffy did to destroy their collars, so he is certainly capable of using Advanced Haki, he has just not realized how to do it.